The invention concerns an electronic temperature controller, particularly for refrigeration appliances, with mains connections and an electronic circuit having a power supply part, a control unit and measuring and setting elements.
Such a temperature controller is known from DE 24 45 172 A1. A small housing arranged on the sidewall of a refrigerator houses the larger part of the electronic circuit. Merely an NTC (Negative Temperature Coefficient) resistor measuring the refrigerator temperature is connected with it via a wire. The resistor is connected in series with a potentiometer, which is manually operable when the refrigerator door is open. The power supply for the electronic circuit is delivered by a 220 V AC mains, which is rectified in a power supply part. The control circuit operates an electronic switch, for example a thyristor or a triac, which is connected in series with a refrigerant compressor.
The NTC resistor is surrounded by a plastic insulating compound, which provides a certain shock protection, but is expensive and increases the thermal time constant of the control, which is often undesirable. The running of the connecting wire to the NTC resistor must be well isolated. The remaining parts of the electronic circuit, except for the rotary knob of the potentiometer are protected against touch by the housing.
DE 31 41 736 C2 discloses a temperature controller comprising an electronic monitoring arrangement and a conventional, non-electronic thermostat. The monitoring arrangement carries low voltage and is galvanically separated from the mains voltage by a transformer. Signals between the two galvanically separated systems are transmitted by means of optocouplers.
The low voltage offers a sufficient shock protection, however requires a transformer, that is, an expensive and large component, which cannot be accommodated in the normal temperature controller housing designs.
Instead of the traditional ways of reaching shock protection, the international standard EN 60730-1, edition 1993-10 (IEC 730-1), Annex H:xe2x80x9cAutomatic electrical controls for household and similar usexe2x80x9d describes a different technique of reaching shock protection. This becomes possible through the use of a protective impedance, which reduces a possible discharge or fault current to maximum 0.7 mA AC (peak value) or 2 mA DC (Chapter H8.1.10.1). The protective impedance is created in that at least two single impedances are connected in series, that is, at least two single impedances between the phase conductor mains connection and the electronics and also at least two single impedances between the zero conductor mains connection and the electronics (chapter H11.2.5).
The purpose of the invention is to provide a shock proof electronic temperature controller needing little space and being inexpensive in production.
According to the invention this task is solved in that the mains connections are connected with the power supply part via protective impedances consisting of at least two single impedances connected in series, and that the electronic circuit is dimensioned for an operating current of maximum 0.35 mA (peak value) when connected to an AC mains and maximum 1 mA when connected to a DC mains.
Due to the shock protection provided by the protective impedances according to EN 60730-1, Annex H, an expensive isolation can be avoided. Sensor elements for evaporator temperature and/or cold room temperature can be placed anywhere. Also the nominal temperature setting arrangement is safe to be touched by the user. In this connection it is necessary for the electronic circuit to be dimensioned so that a very low operating current, which is far lower than the normal values for electronic temperature controllers, will be sufficient.
Another solution of the task, which permits higher operating currents, involves that the mains connections are connected with the power supply part via protective impedances comprising at least two single impedances connected in series, and that the electronic circuit has an earthing connection and is dimensioned for an operating current of less than 0.7 mA (peak value) when connected to an AC mains or less than 2 mA when connected to a DC mains.
It is recommendable that at least the protective impedances, the power supply part and the control part are arranged on a common carrier. The fitting on the common carrier, in particular a printed circuit board, keeps the protective impedances and the other components safely in place, so that small distances will be sufficient. It is possible to accommodate a complete electronic temperature controller in a standard housing, which normally comprises a bimetal setting arrangement and is connected with a fluid-filled capillary tube leading to the evaporator. The individual parts are easy to manufacture, so that in total the electronic temperature controller is inexpensive.
Advantageously, the protective impedances are formed by discrete components, each having a basic substrate on the ends of which connection terminals and between which the at least two single impedances are arranged. The embodiment as discrete component offers the opportunity to provide an adaptation to different mains voltages. The embodiment permits safe functioning of the protective impedances even with the smallest dimensions.
Advantageously, the single impedances are ohmic resistors. These require less space than an inductivity or a capacity.
This particularly applies when the ohmic resistors are applied on the basic substrate as thick film.
It is particularly advantageous for each protective impedance to have three ohmic resistors connected in series. Meeting the dimensioning regulations of EN 60730-1 this gives particularly small component dimensions.
In this connection it is recommended that each ohmic resistor is covered by an isolating layer and is dimensioned so that a voltage drop of maximum 50 V (effective value) will occur at maximum operating current. This is reached at a mains alternating voltage of 240 V (effective value), when three resistors are connected in series in each of the two protective impedances. In this way, small creepage distances and thus also small dimensions can be reached.
In a preferred further embodiment it is provided that the protective impedances are formed by discrete components fitted on the carrier, the live parts being arranged on the surfaces of the components facing the carrier. Thus, the protective impedances are still shock-proof when the housing is removed.
In an alternative embodiment this effect is reached in that the protective impedances are covered by a component, which is also fitted on the carrier.
Preferably, this component is a switching relay. The dimensions of such a relay, including its housing, are sufficient for the covering. When fitted on the common carrier, however, it can also be arranged in a normal housing.
A further alternative involves that the carrier is a printed circuit board and that the protective impedances are arranged inside the printed circuit board. Such a printed circuit board can be made in multilayer technique, the single impedances being arranged between an upper and a lower covering layer.
Additionally, it is advantageous that the carrier has sensor element connections. The sensors, which can be connected here, can be installed without a definite isolation.
It is also recommended that a setting potentiometer is fitted on the carrier. As it is shock proof due to the protective impedances, special measures are not required here either.
In a preferred embodiment it is provided that the control part drives a bistable switching relay. Bistable switching relays cost about 20% more than ordinary monostable relays. However, the latter need a relatively large, constant energy supply to maintain their position, whereas a bistable relay only needs one impulse for the switching.
Expediently, a storage capacitor is allocated to the bistable switching relay. As the current consumption of the electronic circuit is limited due to the protective impedances, the storage capacitor provides that in the switching moment a sufficient current impulse is available for the bistable switching relay.
In a further embodiment of the invention the bistable switching relay and the storage capacitor are also fitted on the carrier. All these parts can be held in a relatively small housing.
Another opportunity of keeping the operating current small exists in that the control part samples the measuring values from the sensors at temporal intervals.
It is also advantageous that each protective impedance has at least two parallel branches, each comprising at least two single impedances connected in series. This results in impedance values permitting an operating current close to the upper limit value.